The present invention relates to a field-emission electron source such as a cold-emission electron source having prospective applications to an electron-beam-induced laser, a flat solid display device, an ultra-high-speed extremely small vacuum element, and the like. More particularly, it relates to a field-emission electron source using a semiconductor which can be integrated and operated at a low voltage and a method of manufacturing the same.
As the progression of semiconductor micro-fabrication technology has enabled the manufacturing of an extremely small field-emission electron source, vigorous research and development has been directed toward the technology of vacuum microelectronics. To implement a high-performance field-emission electron source operable at a lower driving voltage, there has been adopted, e.g., the approach of producing a miniaturized withdrawn electrode and a sharply pointed cathode by using LSI technology.
Referring to FIGS. 19 to 21, there will be described a first conventional embodiment, which is an extremely small field-emission electron source formed by using a silicon substrate and a method of manufacturing the same disclosed in European Laid-Open Patent Publication No. 637050A2.
First, as shown in FIG. 19(a), a silicon oxide film 102 is formed by thermal oxidation on the (100) crystal plane of a silicon substrate 101 made of a silicon crystal, followed by the formation of a photoresist film 103 on the silicon oxide film 102.
Next, as shown in FIG. 19(b), the photoresist film 103 is processed by photolithography to form disk-shaped etching masks 103A each having a diameter of about 1 .mu.m. Subsequently, the pattern of the etching masks 103A is transferred to the silicon oxide film 102 by dry etching for forming disk-shaped elements 102A, followed by the removal of the etching mask 103A.
Next, anisotropic dry etching is performed with respect to the silicon substrate 101 by using the disk-shaped elements 102A as a mask, thereby forming cylindrical elements 104A made of the silicon substrate 101 under the disk-shaped elements 102A. Thereafter, crystal anisotropic etching is performed with respect to the cylindrical elements 104A, thereby forming hourglass elements 104B each composed of a pair of truncated cones with their top surfaces joined to each other and having a side surface including the (331) crystal plane, as shown in FIG. 19(d).
Next, as shown in FIG. 20(A), a thin first thermal oxide film 105 is formed over the surfaces of the hourglass elements 104B and of the silicon substrate 101. Then, anisotropic dry etching is performed with respect to the silicon substrate 101 by using the disk-shaped elements 102A as a mask, thereby transforming the hourglass elements 104B into cylindrical elements 104C with respective hourglass heads.
Next, as shown in FIG. 20(c), a second thermal oxide film 106 is formed over the surfaces of the cylindrical elements 104C with respective hourglass heads and of the silicon substrate 101 so that tower-shaped cathodes 107 each having a sharply tapered tip portion and an extremely small diameter are formed inside the cylindrical elements 104C with respective hourglass heads.
Next, as shown in FIG. 20(d), insulating films 108 and metal films 109 are successively formed by vapor deposition on the disk-shaped elements 102A as well as on the silicon substrate 101 around the disk-shaped elements 102A.
Next, as shown in FIG. 21, wet etching is performed with respect to the second thermal oxide film 106, thereby removing the disk-shaped elements 102A in conjunction with the insulating films 108 and metal films 109 deposited thereon. This exposes the tower-shaped cathodes 107, while forming the metal film 109 into a withdrawn electrode 109A having an inner diameter equal to the diameter of the disk-shaped element 102A.
As a second conventional embodiment, there will be described a method of manufacturing a field-emission electron source using a material having a low work function disclosed in Japanese Laid-Open Patent Publication HEI 6-231675.
Japanese Laid-Open Patent Publication HEI 6-231675 proposes not only the approach of reducing the size of the cathode and improving the structure thereof described in the first conventional embodiment but also an attempt to improve the performance of the cathodes by selectively depositing the low-work-function material on the tip portions of the cathodes. In accordance with the manufacturing method, the formation of the cathodes is followed by oblique vapor deposition for selectively forming the low-work-function material on the surfaces of the tip portions of the cathodes. Thereafter, a thermal treatment is performed for silicidization. Thus, the manufacturing method intends a great increase in the efficiency of electron emission by lowering the work function at the tip portion of the cathode.
As a third conventional method, there will be described a method reported by M. Takai et al. (J. Vac. Sci. Technol. B13(2), 1995, p.441), wherein a porous layer is formed by anodization on the surface of a cathode.
As shown in FIG. 22, a thermal oxide film 106 formed with an opening corresponding to a region in which the cathode is to be formed is deposited on an n-type silicon substrate 101. In the region in which the cathode is to be formed, there is formed an extremely small cathode 107 made of silicon. On the thermal oxide film 106, there is formed a withdrawn electrode 109A made of Nb with an insulating film 108 interposed therebetween.
The surface of the cathode 107 has been anodized by means of an anodizing apparatus as shown in FIG. 23, whereby a porous layer 107a has been formed therein. The anodizing apparatus shown in FIG. 23 comprises: a reservoir 110 for storing a treating agent composed of HF:H.sub.2 O:C.sub.2 H.sub.5 OH=1:1:2; a sample holder 111 for holding a sample 112 disposed in the reservoir 110; a cathode electrode 113; and an anode electrode 114. In the treating agent, a specified current is allowed to flow between the cathode and anode electrodes 113 and 114 provided on both sides of the sample holder 111, while radiation from an excimer lamp is applied to the sample 112, thereby anodizing the surface of the cathode 107. During the anodization, the composition of the treating agent, the amount of current flowing through the treating agent, and irradiation conditions for the excimer lamp are optimized to form the porous layer 107A made of silicon and having a desired configuration and thickness in the surface region of the cathode 107.
The porous layer 107a formed in the surface region of the cathode 107 has numerous rods each having a diameter on the order of nanometers, which have been formed through the formation of numerous holes each having a diameter on the order of nanometers in the porous layer 107a. The numerous rods effectively serve as current emitting sites. This changes the cathode from point-emission type with one emitting site to surface-emission type with numerous emitting sites, resulting in an increased number of electron emitting sites and improved current-emitting property of the cathode.
Although the field-emission electron source according to the first conventional embodiment is operable at a low voltage due to the tower-shaped cathode having a sharply tapered tip portion with an extremely small diameter, it presents the following problem.
In practical applications of a field-emission electron source, stable and uniform emission of electrons is among critical requirements placed on the performance of the electron source.
In the first conventional embodiment, however, the current emitted from the cathode is greatly affected by vacuum atmosphere and the surface state of the tip portion of the cathode during operation, so that the physical property, such as work function, of the surface of the current emitting element is changed during current emission, causing a significant change in operating current. Hence, the requirement of stable and uniform emission of electrons mentioned above has not been satisfied by the first conventional embodiment. The cause of the unsatisfied requirement may be ions resulting from collisions between emitted electrons and a residual gas around the cathode during operation. The resulting ions collide with the tip portion of the cathode and thereby change the surface state of the tip portion of the cathode.
To suppress such current variations, there have been proposed a method wherein cathodes are integrated on a large scale to average individual variations in the quantity of emitted electrons and thereby stabilize the emitted current and a method wherein an additional element having a current suppressing effect, such as a FET, is provided to forcibly suppress current variations. However, the methods incur a significant increase in manufacturing cost because of lower device design flexibility and the necessity for an additional device structure, which presents a serious problem to the practical applications.
The tower-shaped cathode shown in the second embodiment, which has a surface coating film formed selectively of the low-work-function material on the tip portion thereof, has the following problem that, since the current emitted from the cathode flows intensively to the bottom portion of the tower-shaped cathode, high Joule heat is generated in the bottom portion of the tower when operation is performed with a large current. In the case where a current exceeding a maximum permissible value determined by the substrate resistance and the cross-sectional area of the tower is allowed to flow, the temperature of the cathode is raised by the generated Joule heat. If a temperature exceeding the melting point of the material composing the cathode is reached, the melted cathode may destroy the whole device.
Thus, in the second conventional embodiment, the maximum value of the current that can be allowed to flow to the cathode is lowered with increasing miniaturization of the cathode for reducing the operating current, which presents a large obstacle to operation with a large current.
Although the second conventional embodiment has the possibility of solving the problem because of the low-work-function material formed selectively on the tip portion of the cathode by oblique vapor deposition and subjected to the thermal treatment for forming a silicide film on the tip portion of the cathode, it also presents the following problem since the formation of the silicide film involves the process of forming the metal film by vapor deposition and the subsequent reaction process by thermal treatment.
In general, a film formed by vapor deposition is apt to have an unequal thickness over a wafer since a source of vapor is a point source. Moreover, since the subsequent process of forming a silicide film by thermal treatment utilizes a crystal reaction at the interface between the deposited metal and the underlying silicon substrate, the rate of the silicidization process and the quality of the resulting silicide film are likely to vary due to the unequal film thickness and non-uniform temperature, which causes a problem in the formation of the tip portion of the cathode that should be microstructured.
With the microstructured tip portion of the cathode, the radius of curvature of the tip portion is a parameter exerting a particularly great influence on the characteristics of the operating voltage during electron emission. If coefficients of electrostatic focusing are calculated for individual cathodes on the assumption that the structures of the cathodes are the same except for the radii of curvature of the tip portions, the coefficient of electrostatic focusing calculated for the cathode having the tip portion with the radius of curvature of 2 nm is double the coefficient of electrostatic focusing calculated for the cathode having the tip portion with the radius of curvature of 10 nm. In the second conventional embodiment, the radius of curvature of the tip portion of the cathode easily varies by about 10 nm under the influence of variations in the silicide process, resulting in varied device characteristics, which presents a serious problem to the practical applications.
Since the field-emission electron source according to the third conventional embodiment has the porous layer formed on the surface of the cathode, the number of electron emitting sites is increased with the changing of the cathode from point-emission type to surface-emission type. As a result, the electron emitting property of the cathode is improved to a certain degree, but not to a degree satisfactory for the practical applications.
Moreover, since the field-emission electron source according to the third conventional embodiment has the porous layer formed by anodization on the surface of the cathode, improvements have been intended in device characteristics such as operation at a low voltage and an increased current. To positively achieve the effects of reducing the operating voltage and increasing the current, however, a thick porous layer having a thickness of several hundreds of nanometers should be formed on the surface of the cathode. Specifically, in the case where a porous layer having a thickness of 470 nm is formed on the surface, there has been observed the effect of increasing the current which is five to ten times as large as the current flowing in the case where no porous layer is formed.
However, the formation of a thick porous layer having a thickness of several hundreds of nanometers on the surface of the cathode degrades the configuration of the tip portion of the cathode. Although the critical requirements placed on the performance of the field-effect electron source for the practical applications thereof includes uniform electron emission and stable device characteristics in addition to a reduced operating voltage and an increased current, the radius of curvature of the tip portion of the cathode varies in the field-emission electron source according to the third conventional embodiment, which in turn causes the problems of non-uniform electron emission and unstable device characteristics.